After some further testing, I still do not see the value being updated in the case of the partial (but apparently successful) parse. I have added the :default rule you suggest above to two cases. Example 1 has been modified to match all tokens in the grammar( '12' and '3' ). It does return the expected 'name, value' structure. However, Example 2 retains the original string that only matches the first token ('12'). If the parse was successful, I would expect at least the matched tokens to appear in the value data, but it does not appear to exist. I have taken the time to read through the read and value documentation you linked above, but this information does not seem to explain the results I see here (or at least I'm not making sense of it).

use strict;
use warnings;

use Marpa::R2;
use Data::Dumper;

# Example 1
{
    my $g_str = <<'END_GRAMMAR';
:default    ::= action => [name, value]
:start      ::= data
data        ::= '12' '3'
END_GRAMMAR

    my $g_obj = Marpa::R2::Scanless::G->new({ source  => \$g_str });
    my $p_obj = Marpa::R2::Scanless::R->new({
        grammar => $g_obj,
        trace_values => 1,
        trace_terminals => 1
    });

    my $s = '123';
    $p_obj->read( \$s );

    print "EXAMPLE 1 VALUE:".Dumper($p_obj->value)."\n";
}

# Example 2
{
    my $g_str = <<'END_GRAMMAR';
:default    ::= action => [name, value]
:start      ::= data
data        ::= '12' '3'
END_GRAMMAR

    my $g_obj = Marpa::R2::Scanless::G->new({ source  => \$g_str });
    my $p_obj = Marpa::R2::Scanless::R->new({
        grammar => $g_obj,
        trace_values => 1,
        trace_terminals => 1
    });

    my $s = '12';
    $p_obj->read( \$s );

    print "EXAMPLE 2 VALUE:".Dumper($p_obj->value)."\n";
}
[download]

Output:

Setting trace_terminals option
Setting trace_values option
Lexer "L0" accepted lexeme L1c1-2: '12'; value="12"
Lexer "L0" accepted lexeme L1c3: '3'; value="3"
EXAMPLE 1 VALUE:$VAR1 = \[
            'data',
            '12',
            '3'
          ];

Setting trace_terminals option
Setting trace_values option
Lexer "L0" accepted lexeme L1c1-2: '12'; value="12"
EXAMPLE 2 VALUE:$VAR1 = undef;
[download]

In example 2, given there is now a :default rule for returning match data from a structural rule, and that the parse was successful, why is there no output from value?

My guess is that since the 'data' token requires both '12' and '3' to match, 'data' was not successfully matched. This means there is no returned value for the token, resulting in an empty value data structure. But this sounds an awful lot like a grammar parse failure that should result in a ->read failure.

EDIT: As it turns out, I am making the faulty assumption that because the read method did not throw an error, the parse was successful. This is not necessarily true. See my post above Re^3: Marpa -- Partial grammar match does not result in failure